The present invention relates to thermoforming and more particularly to a method and apparatus for vacuum forming thermoplastics.
Thermoforming is widely used in a variety of industries to manufacture products from thermoplastic materials, such as high-density polyethylene (“HDPE”). Many different thermoforming processes are currently in widespread use, including vacuum forming, blow molding and the like. A typical thermoforming process involves the general steps of heating a thermoplastic material above its glass-transition temperature, shaping the heated thermoplastic by drawing the material over a mold, cooling the thermoplastic to a low enough temperature for it to retain its shape and then removing the shaped thermoplastic from the mold.
Vacuum forming is perhaps one of the most versatile thermoforming processes in that it is well suited for use in a wide variety of applications. With vacuum forming, a partial vacuum is used to draw a thermoplastic material against a mold. A conventional vacuum forming apparatus is shown in FIG. 1. The apparatus is configured to thermoform a thermoplastic sheet 18. As illustrated, the conventional apparatus 10 generally includes a movable rack 12 carrying sheet 18 in clamps 11, as well as a mold 14 that is positioned over a vacuum head 16. The undersurface of the mold 14 is shaped to cooperate with the vacuum head 16 to define a vacuum chamber 22. A plurality of vacuum holes 20 extend through the mold 14 from the upper surface to the chamber 22. As a result, operation of vacuum head 16 creates a partial vacuum within chamber 22 that draws air into the chamber 22 through the holes 20. During operation, the rack 12 positions over and parallel to the mold 14. The rack 12 then lowers the thermoplastic sheet 18 onto the mold 14 while a partial vacuum is formed in the chamber 22 by vacuum head 16. The vacuum draws the sheet 18 down over the mold 14 to provide consistent, uniformly shaped articles. The vacuum is retained until the sheet 18 has cooled sufficiently to maintain its shape.
To improve manufacturing efficiency, there is an ongoing effort to reduce the cycle time of thermoforming processes. To date, efforts to reduce cycle time have focused primarily on reduction of the time necessary to heat the thermoplastic sheet beyond its glass-transition temperature. For example, research in this area has lead to the development of a variety of faster heating devices. Despite the focus in the industry on heating time, cooling time is also an important factor in overall cycle time, particularly in view of the significant reductions in heating time. In many conventional applications, the mold 14 includes an internal network of coolant lines 26 that circulate a coolant, such as water, to lower the temperature of the mold 14 and consequently the temperature of the sheet 18.
Conventional coolant line arrangements suffer from several significant drawbacks. First, coolant lines present an obstacle with respect to the formation of the vacuum holes 20 in the mold 14. Conventional molds 14 are manufactured by casting the mold in place about the coolant lines. The vacuum holes are then drilled through the mold at the appropriate locations. Because the coolant lines are entrapped within the mold, the vacuum holes must be carefully position so that they do not penetrate or rupture the coolant lines. This may prevent the vacuum holes from being located as would otherwise have been desired. Second, coolant lines typically do not provide uniform temperature across the mold. Instead, coolant lines generally cause different bands of temperature along the mold 14. For example, areas closer to a coolant line will be cooler than those located farther away. This results in non-uniform cooling along the sheet 18. Third, with complex molds, it can be difficult or impossible to locate cooling lines in certain areas of the mold. This reduces the cooling effect of the coolant lines in such areas.
Other techniques that have been used to reduce cooling time include cooling the sheet with water and nitrogen misting devices. These devices spray a mist of water or nitrogen onto the surface of the sheet after the sheet has been drawn over the mold. Although providing some improvement, these techniques are not without drawbacks. First cooling times associated can vary dramatically with these devices depending on ambient air temperature and humidity. Second, misting devices, particularly nitrogen misting devices, can be expensive to install and operate.